In the English language, yo has become a common interjection that originated decades ago in the Philadelphia region. It is often interchangeable with the word "hey," as in "Yo, what's up? or, "Yo! Wait for me!" While the word can also stand alone as a greeting, like the word "hey," it also has a wide range of meanings that depend on the tone, context, and situation in which it is used. (see examples)

In more recent times the word has spread into hip-hop or gangsta culture. A parody of such usage forms the basis for a comic exchange between the animated characters played by Robert DeNiro and Martin Scorsese (Shark Tale, 2004), which can be viewed here (http://yoism.org/index.php?id=YoST/).

Members of this later cultural scene may claim its origin as theirs. The word's origin, however, clearly predates these recent uses, as is evidenced by the words of Rocky Balboa (played by Sylvester Stallone)

"Yo, Adrienne, it's me, Rocky..." Rocky Balboa (Rocky, 1976)

"Yo, Adrienne. We did it!" Rocky Balboa (Rocky II, 1979)

If someone is bothering you, "Yo!" becomes the equivalent of saying "Hey! Stop it," or "Knock it off!"

If someone accidentially bumped into you, the expression "Yo!" could be interpreted as "Hey! Watch it," or "What the hell!"

If someone did something that amazed you or shocked you, the word yo is like laughing or just an expression of amazement.

Back in the 70's and 80's, the baby boomer Italian's from NYC used "yo" like an equivalent of whistling out loud to get someone's attention-- usually someone at a distance or if you wanted to immediately get attention up close. The word was also preceded with a "hey" in many cases, as in "hey, yo." This was the old-Italian guard usage. When African Americans got a hold of it, "yo" was not only used with the "Italian usage," but also became an abbreviation for the possessive, "your."
Otherwise, see Lili2344 for a history lesson on where the slang originated.